Compensation of thermal nonlinearity effect in optical resonators

Thermal nonlinearity is known to cause bistability in Whispering Gallery Mode (WGM) resonators and to destabilize the red slope of the Lorentzian resonant curve. We demonstrate an optical technique that allows compensation of the thermal effect and forces the resonances to appear linear with both red and blue slopes stable

Microwave synthesizer using an on-chip Brillouin oscillator

Low-phase-noise microwave oscillators are important to a wide range of subjects, including communications, radar and metrology. Photonic-based microwave-wave sources now provide record, close-to-carrier phase-noise performance, and compact sources using microcavities are available commercially. Photonics-based solutions address a challenging scaling problem in electronics, increasing attenuation with frequency. A second scaling challenge, however, is to maintain low phase noise in reduced form factor and even integrated systems. On this second front, there has been remarkable progress in the area of microcavity devices with large storage time (high optical quality factor). Here we report generation of highly coherent microwaves using a chip-based device that derives stability from high optical quality factor. The device has a record low electronic white-phase-noise floor for a microcavity-based oscillator and is used as the optical, voltage-controlled oscillator in the first demonstration of a photonic-based, microwave frequency synthesizer. The synthesizer performance is comparable to mid-range commercial devices

Low-noise Brillouin laser on a chip at 1064 nm

We demonstrate narrow-linewidth-stimulated Brillouin lasers at 1064 nm from ultra-high-Q silica wedge disk resonators on silicon. Fundamental Schawlow–Townes frequency noise of the laser is on the order of 0.1  Hz^2/Hz. The technical noise spectrum of the on-chip Brillouin laser is close to the thermodynamic noise limit of the resonator (thermorefractive noise) and is comparable to that of ultra-narrow-linewidth Nd:YAG lasers. The relative intensity noise of the Brillouin laser also is reduced by using an intensity-stabilized pump laser. Finally, low-noise microwave synthesis up to 32 GHz is demonstrated by heterodyne of first and third Brillouin Stokes lines from a single resonator

Design and characterization of whispering-gallery spiral waveguides

Whispering gallery delay lines have demonstrated record propagation length on a silicon chip and can provide a way to transfer certain applications of optical fiber to wafer-based systems. Their design and fabrication requires careful control of waveguide curvature and etching conditions to minimize connection losses between elements of the delay line. Moreover, loss characterization based on optical backscatter requires normalization to account for the impact of curvature on backscatter rate. In this paper we provide details on design of Archimedean whispering-gallery spiral waveguides, their coupling into cascaded structures, as well as optical loss characterization by optical backscatter reflectometry

Thermal stress in silica-on-silicon disk resonators

The thermal expansion mismatch of thermal grown silica on a silicon wafer is well known to induce compressive stress upon cooling from the growth temperature to room temperature. In this Letter, we investigate how this stress impacts silica disk structures by comparison of measurements with both a finite element and an analytical model. The disk structures studied are also whispering gallery optical resonators, and proper control of stress is critical to obtain high-Q resonances. Based on our analysis, thicker oxide layers and proper control of undercut enable ultra-high-Q optical performance and mechanical stability

Phonon laser action in a tunable, two-level photonic molecule

The phonon analog of an optical laser has long been a subject of interest. We demonstrate a compound microcavity system, coupled to a radio-frequency mechanical mode, that operates in close analogy to a two-level laser system. An inversion produces gain, causing phonon laser action above a pump power threshold of around 50 $\mu$W. The device features a continuously tunable, gain spectrum to selectively amplify mechanical modes from radio frequency to microwave rates. Viewed as a Brillouin process, the system accesses a regime in which the phonon plays what has traditionally been the role of the Stokes wave. For this reason, it should also be possible to controllably switch between phonon and photon laser regimes. Cooling of the mechanical mode is also possible.Comment: 4 pages, 4 figure

Characterization of a high coherence, Brillouin microcavity laser on silicon

Recently, a high efficiency, narrow-linewidth, chip-based stimulated Brillouin laser (SBL) was demonstrated using an ultra-high-Q, silica-on-silicon resonator. In this work, this novel laser is more fully characterized. The Schawlow Townes linewidth formula for Brillouin laser operation is derived and compared to linewidth data, and the fitting is used to measure the mechanical thermal quanta contribution to the Brillouin laser linewidth. A study of laser mode pulling by the Brillouin optical gain spectrum is also presented, and high-order, cascaded operation of the SBL is demonstrated. Potential application of these devices to microwave sources and phase-coherent communication is discussed